1. Field of the Invention
The present invention relates to a signal processing circuit.
2. Description of the Related Art
A full-scale voltage of an AD converter is determined by a product of the number of bits and a voltage of one LSB (Least Significant Bit) of the AD converter. Generally, this full-scale voltage is set based on a reference voltage input to the AD converter (see, e.g., Japanese Patent Laid-Open Publication Nos. 1995-221645 and 2004-72224).
In the AD converter described above, as the reference voltage changes, the full-scale voltage changes. For this reason, generally, such voltage is used as the reference voltage that is of high accuracy and does not change in its voltage level due to ambient temperature, noise, etc.
By the way, for example, a signal from a capacitor microphone such as a MEMS (Micro Electro Mechanical System) microphone changes not only according to audio input to the MEMS microphone but also according to the temperature. For this reason, in the case of processing the signal from the MEMS microphone by the AD converter, the amplitude of the signal input to the AD converter can possibly increase due to a temperature rise, as shown, for example, in FIG. 5. In such a case, if the full-scale voltage is constant, the input signal cannot be quantized with accuracy.